The initial development of the hydrophilic gel that comprises today's flexible lens occurred in 1960 in Europe by Professor Otto Wichterle and Dr. Drahoslav Lim. The importance of the structural similarity of the gel material to living tissue in eliminating the incompatibility between foreign body and tissue was stressed. The use of this material for contact lenses followed. Hydrophilic lenses became commercially available in Europe during the 1960's.
The Bausch and Lomb lens, "Soflens" was the first type of such soft contact lens to be approved by the Food and Drug Administration in the United States. The lens was approved for cosmetic purposes and served as the guideline for other lens manufacturers. A list of lenses approved by the FDA appears in the February 1980 issue of Contact Lens Forum. Aphakic lenses, such as the "Permalens" manufactured by the Cooper Co., Mountain View, Calif., the "Hydrocurve II" lens manufactured by Hydrocurve, Inc., San Diego, Calif., and the "Sofaulon" lens manufactured by Hydro Schulte are such types of lenses which have been approved by the FDA for aphakic patients and for cosmetic use. Such types of "soft" contact lenses are generally formed of a cross linked polymeric material capable of forming a three-dimensional matrix which permits water absorption, thereby allowing the lens to be applied to the eye.
The polymeric compositions used in such soft, high water content lenses include the following:
Polymacon (38.6% water): the homopolymer including hydroxyethylmethacrylate (which contains the hydroxy radical which makes the material hydrophylic) and ethylene glycoldimethacrylate (which acts as the cross-linking agent). The structure of the lens material is a three-dimensional network of chain-like macromolecules joined by cross-links. The HEMA unit forms the chains and ethylene glycoldimethacrylate forms the cross-links. The number of cross-links is small compared with the number of repeating units on the main polymeric chain.
Hefilcon A (45% water): the random copolymer of 2-hydroxyethylmethacrylate and N-vinyl-2-pyrrolidone. Ethylene glycoldimethacrylate forms the cross-links. There is approximately one cross-link for every seventy monomer units. The hydrophilic properties of the material are due to the free hydroxyl and carbonyl groups present in the structure.
Bufilcon A (45% water): a hydrophilic random copolymer of 2-hydroxyethylmethacrylate, N-(1, 1-dimethyl-3-oxobutll)-acrylamide, and methacrylic acid. The structure is a three-dimensional network of copolymer chains joined by trimethylolpropane trimethacrylate cross-links at a density of about one cross-link for every 1400 monomer units.
Tetrafilcon A (42.5% water): a random terpolymer of 2-hydroxyethylmethacrylate, N-vinyl-2-pyrrolidone, and methylmethacrylate. The polymer is a three-dimensional network of terpolymer chains joined by divinylbenzene cross-links.
Ocufilcon (46% water): 2-hydroxyethylmethacrylate and methacrylic acid cross-linked with ethylene glycoldimethacrylate.
Dimefilcon A (36% water): a hydrophilic copolymer of 2-hydroxyethylmethacrylate and methylmethacrylate, cross-linked with triethylene glycoldimethacrylate.
Vifilcon A (55% water): a soft hydrophilic copolymer of 2-hydroxyethylmethacrylate and povidone, USP. The chemical name is: Poly (2-hydroxyethylmethacrylate-co-ethylene dimethacrylate-co-methacrylic acid-g-povidone).
Droxifilcon (46% water): a random copolymer of 2-hydroxyethylmethacrylate and methacrylic acid modified with polyvinylpyrrolidone. The polymer is a three-dimensional network of copolymer chains cross-linked by triethylene glycoldimethacrylate.
Deltafilcon A (43% water): a cross-linked 2-hydroxyethylmethacrylate modified with isobutyl methacrylate.
Etafilcon A (43% water): a random copolymer of 2-hydroxyethylmethacrylate and methacrylic acid cross-linked with 1,1,1-trimethylolpropane trimethacrylate.
Phemecol or phemfilcon A (30% water): a cross-linked three-dimensional polymer network of 2-hydroxyethylmethacrylate and a small percentage of cross-linking monomers.
See: A Clinical Guide to Soft Contact Lenses, Spinell, M., 1979, at pages 13 et seq.
After the introduction of the soft lenses, it was noticed that deposits were being formed on the lenses. Patients had complained of lens discomfort and blurred vision. In most instances, the problem was remedied by fitting the patient with a new lens. This was an expensive and tedious solution to a cleansing problem. At approximately the same time, many laboratories initiated studies to determine the nature of the surface deposits that had formed on the lens after prolonged use. It had become apparent that the deposits needed to be identified in order to develop specific prophylactic or restorative techniques. Most of the studies revealed the predominant presence of proteins, especially lysozyme in such deposits; however, most of these same studies also revealed the presence of mucin, lipid, calcium, iron, and perhaps other debris.
In these studies, the findings differ as to the identification of the surface deposits. One particular study indicates that there are three different types of deposits: crystalline, proteinaceous, and granular. A Clinical Guide to Soft Contact Lenses, supra, at page 193. The primary source of the deposits are traceable to the tear constituents.
The proteinaceous deposits develop from constituents in the tear film that are secreted by the surrounding glands. They are solidified by heat in a low pH saline and bind with the actual lens material. An enzyme cleaner is available commercially from Allergan Pharmaceuticals, Irvine, Calif. which consists of papain enzyme and is useful in moderately removing this type of deposit. However, the use of a papain enzyme cleaner results in surface changes in the lens which, in turn, enhance the deposition of debris on further use of the lens.
The granular deposits, which actually grow into the lens matrix, have a gelatinous appearance. It is believed that these deposits result from a combination of stress, dryness, and lipid deposition.
Crystalline deposits are generally calcium or magnesium salts or other mineral type deposits and occur when a lens is stored in a high PH solution.
Hence, not only are such deposits adhesive to the surface of the lens, but such deposits also penetratingly adhere to the lens in a manner which may be characterized as a "growth" within the intestices of the polymeric matrix material from which the lens is formed.
Deposits which develop after use of a soft contact lens interfere with visual acuity and cause patient discomfort, ocular infection, and possibly allergic conjunctivitis. Deposits encourage dehydration, which in turn influences vision, comfort, and corneal integrity. The deposits also interfere with heat and chemical sanitization procedures. In this regard, it has been considered that although the majority of patients can wear extended wear contact lenses safely, the problem of calcium and protein deposits is a major stumbling block of long term successful wear of extended wear contact lenses.
Various cleansing solutions for soft contact lens care were developed to obviate the need for the wearer frequently to replace lenses because of the accumulations of debris on the lens. There are several solutions on the commercial market and several others are described in the patent and technical literature.
These solutions contain a variety of chemicals designed to aid in their primary function of cleansing. The additional chemicals may act as buffers, preservatives, and wetting agents. These constituents are present to create an optimum environment (pH) in which the chemicals can act, to keep the solution stable, to insure ocular comfort, or to disinfect the solution. Since soft lenses are hydrophilic, it is essential that the lens solution contain ingredients that will keep them wet and lubricated.
The cleansing solutions may be generally classed into detergents, surfactants, salines, hydraters, and special purpose cleaners. The literature, however, in describing the several solutions, emphasizes that none is completely satisfactory. See A Clinical Guide to Soft Contact Lenses, supra, at pages 185, 186 and 193-195.
There is described in U.S. Pat. No. 3,910,296 an aqueous solution for the cleaning of lenses containing a proteolytic enzyme, including the protease identified above as employed in a commercially available product, papain. Additionally, the solution described in the patent contains a non-toxic amount of a sulfhydryl group containing compound. A later filed and issued U.S. Pat. No. 4,096,870 discloses a similar solution using papain that is formulated into tablets using sodium chloride and boric acid as binding agents.
The enzymatic cleaner, papain, is a proteolytic enzyme derived from the dried and purified latex of the pawpaw tree. It is purported to be effective in removing moderate amounts of proteinaceous deposits from lens surfaces. A kit is commercially available from Allergan Pharmaceuticals, Inc. Irvine, Calif. that consists of two vials and a stabilized papain enzyme tablet. Such kits are provided for use with Bausch & Lomb "Soflens" polymacon contact lenses. In use of the kit, a specific amount of distilled water is poured in each vial and a tablet is allowed to dissolve therein. The lens is placed in the solution for six to twelve hours. It was noted that the solution took on the odor of hydrogen sulfide (rotten eggs). The lens when removed is washed off with saline. The saline may remove the milky white film that sometimes remains on the lens.
Although the papain enzymatic cleaner is an advance in the art, the advance is not sufficient. Initially, the effectiveness of the enzyme papain is limited to removing proteinaceous deposits but is not effective against non-proteinaceous deposits. Thus, although the proteinaceous deposits may be the predominant debris to be formed on the lens, the many studies have concluded that there is a significant amount of mucin, lipid, and calcium debris that will accumulate on the lens through prolonged use, and this debris is not removed by papain.
Another self-destructing factor found with the use of the enzyme papain is that in the process of removing surface deposits, the enzyme simultaneously creates new lens surfaces with small pits which actually encourage new deposit formations. That is, the pits are irregular and roughened to cause the debris to more readily adhere therein after the patient resumes wear of the cleansed lens.